In order to precisely read/write an optical disk such as a CD, a DVD, or a BD (blue-ray disc), it is necessary to (i) cause light emitted from a light source to enter an objective lens so that a very small beam spot is formed on the optical disk, and to (ii) cause the beam spot to follow a desired recording track while maintaining a shape of the beam spot.
One of the components essential for realization of precise read/write of an optical disk is an aperture, i.e., an opening provided in an objective lens holder. An arrangement of the aperture is described, for example, in Patent Document (Japanese Unexamined Patent Publication 329508/1996 (Tokukaihei 8-329508; published on Dec. 13, 1996)).
FIG. 8 is a side cross-sectional view schematically illustrating an arrangement of a conventional objective lens holder according to Patent Document 1. As illustrated in FIG. 8, the objective lens holder 26 includes a cavity 27. The cavity 27 guides, to an objective lens 21, a light beam emitted from a light source. On one side of the objective lens 21 facing a light source, an aperture 41 is provided.
As illustrated in FIG. 8, the aperture 41 is provided so as to be sandwiched between protrusions that stick out toward a center of the cavity 27 with respect to internal wall surfaces 27a of the cavity 27. The aperture 41 shields the light beam whose diameter is larger than an effective diameter 61 of the objective lens 21. Thus, the objective lens holder 26 is arranged such that the aperture 41 guides, to the objective lens 21, only the light beam whose diameter corresponds to the effective diameter of the objective lens 21.
Further, the aperture 41 makes it possible to shape, into a circular light beam, the light beam entering the objective lens 21 from the light source. Furthermore, even when the objective lens 21 is shifted, the light beam can be constantly shaped into the effective diameter 61.
However, the conventional objective lens holder 26 illustrated in FIG. 8 suffers from such a drawback that the aperture 41 is low in strength.
In recent years, an optical disk read/write device has been required to have a higher transfer rate. In order to realize the higher transfer rate, an objective lens driving device of the optical disk read/write device is required to control the objective lens at high frequencies, i.e., to realize a wide-bandwidth servo characteristic.
In order to realize the wide-bandwidth servo characteristic, a driving unit including the objective lens or objective lens holder needs to have a resonant frequency as high as possible. That is, there is a demand for such an arrangement that, even when the objective lens or objective lens holder is driven at high resonant frequencies, the light beam having an appropriate diameter that corresponds to the effective diameter of the objective lens enters the disk and the light beam reflected from the disk is appropriately detected by a photodetector. In order to realize such an arrangement, the objective lens holder needs to be highly rigid.
However, the objective lens holder 26 illustrated in FIG. 8 is arranged such that the aperture 41 is provided so as to be sandwiched between the protrusions that stick out toward the center of the cavity 27 with respect to the internal wall surfaces 27a of the cavity 27. With such an arrangement, the aperture 41 sandwiched between the protrusions is too weak.
With such an arrangement, the aperture itself becomes a source of resonant oscillation. As a result, the light beam having the appropriate diameter that corresponds to the effective diameter of the objective lens cannot enter the disk. Further, the oscillation of the aperture generates an unnecessary light beam added to the light beam returning to the photodetector. This generates resonant oscillation (phase distortion) in a control signal (feedback signal) of an actuator. As a result, control becomes impossible. Therefore, the conventional objective lens holder suffers from a drawback of being incapable of performing high-speed read/write operations.